The purpose of this grant is to investigate the efferent innervation of the peripheral vestibular apparatus. Knowledge of the neurotransmitters and the projections from the brainstem efferent neurons to the peripheral labyrinth will help us understand the role of the vestibular efferent system in health and disease. Recent studies have suggested that neurons in the vestibular efferent nucleus are heterogeneous in their transmitter composition and in their peripheral targets. Such heterogeneity changes our long-held view that vestibular efferents are a nonspecific system. The goal of this study is to define subpopulations of efferent neurons in the chinchilla based upon their brain stem locations, their projection patterns, their peripheral endings, their neurotransmitters, and their receptor subtypes. Specific aims are: 1) to characterize anatomical and neurochemical subpopulations of brainstem efferent neurons by combining retrograde and anterograde tracers with transmitter immunohistochemistry, and 2) to determine details of the morphological terminations of efferents in the periphery, at both the light and electron microscopic levels, and details of their neurochemistry at the molecular level by determining their transmitter receptor subtypes. Efferent neurons in the brainstem are identified by means of retrograde tracer injections into the peripheral vestibular apparatus. They are simultaneously characterized immunohistochemically with antibodies to several putative neurotransmitters (acetylcholine, calcitonin gene- related peptide, nitric oxide synthase, met-enkephalin, adenosine triphosphate, and others). In the periphery, the distribution of receptor sub-types for peptidergic, purinergic and muscarinic transmission will be examined in an attempt to specifically understand the slow response of afferents to efferent stimulation. Vestibular afferents are identified by means of extracellular horseradish peroxidase injections or calretinin immunohistochemistry. The innervation patterns of efferent terminals in the periphery is determined by injections of biotinylated dextran amine centrally. Electron microscopic immunohistochemical methods are used to characterize chemically distinct efferent boutons according to their terminations in the sensory epithelium, that is, by the class of afferents (calyx, dimorphic or bouton) and by the region (central or peripheral zones) that they innervate. The intent is to produce a body of knowledge about the structural basis of the efferents, and their relationship to the afferents, from which physiologically and pharmacologically testable hypotheses can be derived.